1. Field of the Invention
The invention relates to chromium electroplating, and more particularly to improved electrolytic bath compositions and methods therefor and substrates coated electrolytically with chromium.
2. Prior Art
Present day commercial chromium plating processes are based on the electrolysis of chromium trioxide, CrO.sub.3 (chromic acid) solutions containing small amounts of a catalyst, e.g., sulfuric acid, fluorides, etc. In these commercial processes for the production of bright plates of acceptable quality the current density and temperature during plating must be closely controlled. Even when closely controlling the current density, temperature and other variable parameters such as chromic acid-catalyst ratio, etc., the throwing power of the plating bath is very low as compared to other metal plating processes. Because of the poor throwing power it is often necessary to provide anodes conforming to the shape of the object to be plated. Moreover, the current efficiency of commercial plating baths is usually no greater than 8-12% and under the most optimum condition only about 15%, possibly as high as 20% in rare instances.
Also, objectionably large volumes of oxygen and hydrogen are given off during conventional plating operations, resulting in a highly noxious and corrosive spray of chromic acid over the plating bath during the plating process. Moreover, large amounts of chromic acid are lost by drag-out and spray due to the necessity for employing rather large amounts of chromic acid in commercial baths.
In addition, conventionally electroplated chromium deposits have hardness values unacceptable for many applications, i.e., on the order of 800-950 Hv (Vickers Hardness). Also, at elevated temperatures the hardness values of conventionally plated chromium layers decreases, e.g., from 900 Hv to 400 Hv at 500.degree. C., thereby decreasing the wear resistance of conventional plated substrates.
Finally, conventional chromium plating methods result in a large number of relatively large cracks in the plated layer rendering it subject to corrosion.
It is an object of the present invention to provide an electrolytic bath composition and electroplating method for electrodepositing chromium layers having exceptionally high hardness values.
It is a further object of the present invention to provide an electrolytic bath composition and electroplating method for electrodepositing chromium at higher current efficiencies and for producing chromium layers having a higher degree of brightness and less subject to corrosion of the underlying substrate.
It is a further object of the invention to provide novel substrates electrolytically plated with chromium.